[linux-audio-dev] 2-channel stereo compatible ambisonics...

Fernando Lopez-Lezcano nando at ccrma.Stanford.EDU
Thu Mar 15 20:59:40 UTC 2007

On Wed, 2007-03-14 at 11:10 +0100, Joern Nettingsmeier wrote:
> Steve Harris wrote:
> > 
> > On 13 Mar 2007, at 09:15, Valent Turkovic wrote:
> > 
> >> Hi,
> >> is it possible to record multi channel speech audio and down sample it
> >> 2 stereo channels BUT with horizontal audio positioning of each audio
> >> channel ie. like dolby surround effect.
> > 
> > Sure, but you might be disappointed by the results, Pro Logic is not 
> > very sophisticated (eg. there's no separate rear left and right channel, 
> > and the bandwidth is limited). Also the only free software 
> > implementation that I know of is not state of the art, partly due to 
> > patent problems, and partly due to lack of interest.
> > 
> > There's a LADSPA plugin: 
> > http://plugin.org.uk/ladspa-swh/docs/ladspa-swh.html#id1401
> > which does Pro Logic matrix encoding, you will need some other software 
> > to mix your independent streams into LCR channels (you can do it by ear 
> > with a 3+ variable send bus mixer), but once you have that you can feed 
> > it to the plugin and get a pro logic compatible stream out.
> > 
> > PS ignore the documentation, it has been tested, and it does work, just 
> > not brilliantly. You will get better results if you EQ the mics a bit to 
> > cut the level of the highs and lows a bit, especially in the rear channel.
> > 
> > You would get much better results with an ambisonic stream, but I don't 
> > /think/ you can make stereo-compatible ambisonic streams, and not many 
> > people have ambisonic decoders. Other people on this list know a lot 
> > more about ambisonics though.
> i'm very new to ambisonics, so i may be wrong, but i think there are 
> indeed stereo compatible first-order ambisonics streams. iirc, the 
> format is called UHJ, and it matrixes WXY (lossily) into L/R (i.e. no 
> height information).
> i've changed the subject to bait fons :-D
> if we're lucky he knows an implementation of 2-channel UHJ.
> i found this article: http://www.ambisonic.net/ambi_AM91.html
> but it does not give details as to how the matrix encoding is accomplished.

There's a supercollider implementation in the joshugens packages
(UHJ.sc). It is meant to be played in stereo (ie: you "sorta" decode
ambisonics b format to stereo). 

It's here, you can download the source from svn:

-- Fernando

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